SUMMARY Stroke represents a major health issue not only as a cause of death but because most people survive their first stroke, leaving victims permanently disabled. With an aging baby boom generation in the United States, incidence of stroke and costs to society propose to reach epidemic proportions within the next two decades. While prevention efforts have significantly reduced lethality, few effective treatment options exist to provide patients with a means of recovery, and none exist to address the root cause of the problem, loss of brain tissue. Recently however, successful outcomes from cell replacement therapies in pre-clinical studies have led to a number of clinical trials which have demonstrated safety and efficacy for a variety of cell types. Interestingly, many of the mechanisms attributed to transplanted mediated recovery converge on the activity- regulated release of paracrine factors from transplant to host. In the current study, we propose first to validate that transplanted human pluripotent stem cell-derived neurons (hPSNs) improve behavioral recovery in a mouse model of focal ischemia that is amenable to mechanistic and manipulation studies (Specific Aim 1). We will then use this model to test whether optogenetic stimulation of hPSNs can augment behavioral recovery from stroke (Specific Aim 2). Lastly, multiple lines of evidence suggest that functional integration of transplanted cells with host tissue is critical for long-term benefits of cell replacement. In support of this, we have recently demonstrated in uninjured animals that human embryonic stem cell-derived neurons (hPSNs) can functionally integrate with host circuitry after transplantation, and can cause changes to overall excitability in vitro. In the Specific Aim 3 we will test whether and how transplanted hPSNs are functionally integrating with host circuits in vivo to understand how we might augment future intervention strategies. The impact of answering these questions proposes to improve efficacy of cell-based therapeutic interventions.